19. Design of the x86 CPU Instruction Set

• Instruction set architecture (ISA) is an important design issue for CPU designers:

  • caches, pipelining, superscalar implementation can be added to the design later, but

  • it is very difficult to change the CPU instructions once in production and people writing software...

    • ...LOOP instruction on the x86 CPU sees very little use in modern high-performance programs.

• Challenges:

  • Expandability is a big concern (predicting future needs.)

  • Legacy support is almost the opposite of expandability.

• Each instruction requires some number of transistors on the CPU's silicon die.